Rotator cuff tear reaching the superior half portion of the humeral head causes shoulder abduction malfunction.

Abstract

To examine the biomechanical properties governing posterosuperior rotator cuff (RC) tear progression and dynamic shoulder abduction function, in the absence of excess loading. Twelve freshly frozen cadaveric shoulders were evaluated via an established dynamic shoulder abduction stimulator. The shoulder abduction functions were primarily evaluated using subacromial contact pressure (SACP) during an abduction procedure, and subsequent middle deltoid force (MDF) under 5 conditions: (1) intact, (2) anterior 1/3 posterosuperior rotator cuff (PSRC) tear, (3) anterior 2/3 PSRC tear, (4) entire PSRC tear, and (5) global RC tear (tear involving the entire superior RC). No obvious differences were observed in the peak MDF required for abduction, and in the peak SACP among the four PSRC tear statuses (49.8 ± 9.2N, 0.39 ± 0.05mPa [1/3 PSRC tear]; 49.3 ± 6.8N, 0.40 ± 0.06mPa [2/3 PSRC tear]; 51.6 ± 7.0N, 0.44 ± 0.08mPa [entire PSRC tear]), as well as intact statuses (48.3 ± 9.8N, 0.40 ± 0.05mPa). However, significant elevations in the peak MDF and peak SACP levels were observed among the four PSRC tear statuses and global RC tear (68.1 ± 9.3N; 4.12 ± 1.50mPa, P < 0.01). In the absence of excess loading, the biomechanical function of the shoulder was not impaired by a simple PSRC tear. However, once the tear size reached the half superior portion of the humeral head, the humeral head migrated to the surface of the subacromion, and this action markedly decreased shoulder abduction function.